Lime Specification

Why?

Lime is fussy.

Mortars, plasters and concretes with lime as their predominant binder have to be tailored to purpose or they can cause other problems or exacerbate existing ones. 

Factors to take into consideration include 

• Building material eg. type of brick or stone. Very important as certain types can create a chemical reaction with the mortar. 

• Location on the building eg. below ground level. Very important when trying to control moisture.

• Local environmental conditions eg. house is sheltered. Important in some circumstances as it could easily lead to damp.

• National environmental conditions eg. Exposure rating. Very Important as moisture can build up even with lime applications over time. 

• Method of construction eg. is it a Victorian brick built vs solid wall stonemasonry. Important for deciding what really needs replacing.

• Lime eg. putty, NHL, hot, hydraulic hot, natural cement, etc. Important for functionality of predominately older buildings.

• Aggregates eg. sharp or round sand. Very important for functionality. 

• Type of sand in conjunction with lime eg. Mg lime with alumni-silicate. Vitally important and woefully ignored.

• Application method eg. hand or trowel applied. Moderately important in regards to aftercare and ease of application. 

• Time of year eg. winter or summer. Moderately important in regards to aftercare.

• Aesthetics eg. what colour. Unimportant and shouldn't impact any other decision. Design the mortar and then tweak it within that remit. Not add black volcanic sand. because you want to match coal flecks, Go and get some bloody coal, hit it with a hammer and sieve it... Everything original is available with enough effort in the amounts we need 99/100 times. And the answer is nearly always the easiest one eg. your garden or the nearest sand quarry/producer or water source. Obviously no-one has gone around wholesale removing all the subsoil/aggregates in the last hundred years... Ironically there is a sand shortage but that's globally and a much longer conversation that has little to no bearing on traditional mortars. 

When specifying lime there are a wide variety of additives that can be added for increased functionality far beyond that which can be achieved by just mixing washed aggregates with pure or hydraulic lime. An incredibly basic specification which will produce a very limited range of crystals with only one method of production, hydration. A traditional mortar will have a far greater range of crystals produced through chemical and biological methods with other materials improving its ability to do so. This results in a mortar which is stronger whilst remaining within the strength range of traditional mortars, more resistance to water ingress without compromising it's ability to breathe, improved self healing capacity, improved resistance to sulfate and nitrate damage, improved workability and reduced aftercare. This is the reason Roman concrete is so effective, it does more of the same thing, better. Another observation about England in the Dark Ages perhaps not being the best source of how to do things other than burn witches and go crusading? 

Here are a few examples of different mortars and their potential uses:

Lime and brick; an hydraulic mortar with an upper strength limit and an excess of setting methods specified for damp areas or challenging environments with a high risk of contamination. Potentially very fast setting with immediate high strength gains. Used for damp applications and persistently wet areas by hot mixers. 

Lime and chalk; a weak mortar with excellent drying action suitable for milder climates that have an excess of damp. Used in the SE, especially on timber frame buildings. And because I'm Northern, when I say the SE, I mean everything from Birmingham diagonally downwards. Wessex. 

Lime and granite dust; a very strong, fast setting hydraulic mortar. Cheap to produce. Suitable for civils applications. Granolithic mortar is commonly used with cement as a binder for modern civils applications. 

Lime and sand: dependant upon the sand specified, wide range of potential uses but generally a very good all round mortar which allows for less lime to be used that a stone dust. Most common lime mortar specification used by a lot of lime specialists and other low skilled lime users like property developers. Washed sand with a low amount of fines is a lot more predictable and a safer choice so is preferred over other aggregates which I think just confuse most lime suppliers. This is currently the most popular look for lime mortar too, cream lime and a gritty sand with exposed aggregates. Idiot peasant mortar... 

Lime and earth; high potential to go wrong but can be very effective when specified well. Good for above ground works. Used for Cob repairs and Medieval properties among others. 

NHL and sand; varying strengths but very dependant upon the brand of lime. Can be good for certain types of stone which are susceptible to damage from lime leaching. Also suitable for civils applications but less freelime would be preferred in that scenario too. Used by people ticking the lime box and by high end conservators who understand where it should be specified. Over used as it's as close a match to working with cement as you can achieve with lime. Easy to specify and predict, also appeals to the turn it up to 11 ideology within the modern construction industry. 

Lime and limestone dust: can be very effective for a wide varety of applications but can be as unpredicatable as NHL for the same reasons. Potentially high amounts of dormant bacteria which could be revived as well as pozzalonic material, nitrates, and silicates. Mason's mortar. Also a common aggregate used in areas with limestone and no easy access to sand.

Lime and crushed sandstone: another potentially excellent combination, especially when specifying wet pit sand or crushed stone from a banking shed. Mason's mortar used by stonemasons and for stone house repairs. Obviously you want to use exactly the same type of stone as the property is built from and by a stonemason's standards. From the same bloody seam in the same quarry NOT just sandstone or limestone of the same colour. 

Lime and slate dust: high levels of bacteria, resulting in fast strength gain and a strong mortar when fully cured. Used for the colour and as a cheap local aggregate.

Lime and recycled aggregates; not a good idea as they could contain high levels of salt from the building units, concrete and mortar crushed to produce them. Lime mortar does sacrfice itself by also taking on salt until it can't breathe anymore so is it really a good idea to use that or concrete which has high levels of sulfates in it? Nope. Used by unsrupulous lime suppliers in limecrete, not cool as the base of walls already have high levels of salt in most cases. Sulfate; gypsum formation etc. Rising damp, lol AKA hygroscopic salt content at the wall base. 

Lime and perlite: very good choice for insulative and lightweight qualities. Increased bacteria. Used by a lot of lime workers when hot mixing insulative plasters. Mildly pozzalonic due to being a alumi-silicate so the grade is important.

Lime and unwashed beach sand: a risky choice that needs to be tested but potentially very effective as it could contain high levels of beneficial bacteria, nitrates and silicates. Silver sand has been used straight form the beach on the Wirral to produce some very effective basic mortars. 

Lime and marble or dolomite dust: excellent top coat mortar. The dust produces a fast setting and easy to work with mortar which has a very close finish and the potential to be buffed more so than other aggregates.

Lime and coal or ash: this is a hybrid lime/gypsum mortar which I believe was spceified due to the availability of the materials and/or the colour rather than any desire to produce certain results as I can't discern how they would be beneficial in the UK. Very lightweight, blue black coloured and fast drying. Can be exceedingly quick to achieve initial strength, so very hard to control the shrinkage on certain substrates.